5S DNA codes for the small molecular weight 5S ribosomal RNA. In Xenopus this DNA occurs in tandem gene families. One kind of 5S ribosomal RNA is synthesized in oocytes while another is synthesized in somatic cells. These genes differ by several base substitutions and comprise separate gene families under separate control. The transcription unit, that is the region of this DNA which is transcribed into RNA in the living cell, is relatively simple, only 120 base pairs in length. The form of RNA polymerase which transcribes this gene in vitro has been identified as form III. The mature RNA transcript does not appear to be derived from a high molecular weight precursor. Using this simple system in which the purified genes are available, we hope to reassemble the macromolecules which enable this DNA to be transcribed with fidelity in the living cell. In this way we hope to elucidate the control mechanisms of an animal gene.